Current-mode detectors, operating in the photoconductive mode, are generally used whenever the flux of impinging photons is too intense to allow the counting of single photons. This is generally the situation in X-ray imaging. In such situations, the practical method for measuring the intensity of the radiation is to measure the average current stemming from charges created by absorption of a large number of photons. For example, the detectors presently used by Computerized Tomography (CT) imaging systems are scintillators coupled to photodiodes, designated hereinafter SPD's. Each X-ray photon absorbed within the scintillator creates a pulse of light, which is then detected by the photodiode and converted to multiple electron-holes pairs, with the number of those pairs being proportional to the energy of the absorbed X-ray photon. These charges are then sampled over a timeframe which is much longer than the average time between individual events of X-ray photon absorption within the scintillator. The read-out signal is thus proportional to the average current from the photodiode, which is proportional to the X-ray flux hitting the scintillator.
Two decades ago, there was already a trend in CT imaging technology to replace the SPD by all-semiconductor detectors. One of the first such examples is described in the article by P. A. Glasgow et al, entitled “Aspects of Semiconductor Current Mode Detectors for X-ray Computer Tomography”, published in IEEE Transactions on Nuclear Science, Vol. NS-28, pp. 563–571, February 1981. Such detectors have the inherent advantage of operating in a direct conversion mode, i.e., the X-ray photon absorbed within the semiconductor volume is directly converted to electron-hole pairs. Such a direct conversion is an order-of-magnitude more effective than the previously used indirect conversion process of X-ray photons to light within the scintillator, and light to electron-hole pairs within the photodiode. For instance, in a detector made of CdTe or CdZnTe (CZT), the number of electron-hole pairs created by an absorbed X-ray photon of energy E0 is approximately E0/4.4 eV, whereas in a conventional SPD detector, the number is in the range typically of from E0/30 eV to E0/60 eV only. Semiconductor detectors are not only more efficient, but they also allow the fabrication of arrays of detectors over a monolithic slab of the semiconductor, with pixels of desired dimensions, especially of very small dimensions which are practically impossible to fabricate in conventional SPD structures. This is a very important advantage of semiconductor detectors over SPD detectors, since the trend in CT imaging technology is presently for many more channels of detection, using much smaller detectors to allow better spatial resolution. This trend is only feasible currently by replacing the SPD detectors with semiconductor detectors such as CdTe or CZT.
Such semiconductor detector operate in the current-mode by utilizing the photoconductive effect. When the X-ray flux is absorbed within the photoconductor (PC), the conductivity of the semiconductor essentially changes from its dark-value, determined by the thermal excitation of electrons within the bulk of the PC, to a higher conductivity, determined by the higher density of electrons created by the absorbed X-ray photons. Since the PC is kept under a bias voltage between two electrodes contacting its bulk, this change in conductivity is translated into a change in the resultant current, from that of the dark-current to that of the photocurrent.
It has been shown in an article entitled “Possible use of cadmium telluride for detection of pulsed X-rays in medical tomography” by E. N. Arkad'eva et al., published in Soviet Physics—Technical Physics, Vol. 26, pp. 1122–1125, September 1981, that for the PC detector to behave in an optimal way and to exhibit correct temporal behavior, the X-radiation should impinge on the PC detector in a direction perpendicular to the electric field established within the PC. If the X-radiation hits the PC detector parallel to the electric field, i.e. through the cathode or the anode, there will be temporal overshoot of the PC detector current in response to the incidence of an X-ray pulse. Furthermore, the PC detector photocurrent in this parallel mode is considerably smaller than that in the perpendicular mode.
However, according to the current state of detector technology, perpendicular-mode detectors can be fabricated only as rods, with the electrodes on the narrow sides thereof, and pixellated only in one dimension, namely the length of the rod. Consequently, one of the inherent advantages of using a semiconductor detector, namely, the possibility of fabricating two-dimensional pixellated monolithic arrays, cannot be realized. Detectors have been described in which one-dimensional arrays, i.e. pixellated rods, are stacked side by side to form a two dimensional array, but such an array is difficult to fabricate, and is thus costly. There therefore exists an important need for a two-dimensional pixellated monolithic array, capable of being used in the perpendicular mode, with the radiation impinging on the array in a direction perpendicular to the direction of the application of the bias field.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.